Added SDL_GetTextureDXGIResource() to get the DXGI resource associated with a render texture.

Also switched the D3D11 and D3D12 renderers to use real NV12 textures for NV12 data.

The combination of these two changes allows us to implement 0-copy video decode and playback for D3D11 in testffmpeg without any access to the renderer internals.

src/dynapi/SDL_dynapi.sym

@@ -907,6 +907,7 @@ SDL3_0.0.0 {
     SDL_SetAudioStreamFrequencyRatio;
     SDL_SetAudioPostmixCallback;
     SDL_GetAudioStreamQueued;
+    SDL_GetTextureDXGIResource;
     # extra symbols go here (don't modify this line)
   local: *;
 };